A method for identifying characteristics of single-phase grounding faults and selecting phases

Abstract

The present invention discloses a single-phase grounding fault feature recognition and phase selection method. In the process of starting judgment of a single-phase grounding fault, a zero-sequence voltage positive sequence is constructed to obtain the first similarity of the zero-sequence voltage positive sequences between two branches, and the zero-sequence voltages of the corresponding branches are compared pairwise to obtain zero-sequence voltage shape difference values ​​to construct the zero-sequence voltage variation value of the fault line, and whether a branch is started is determined according to the zero-sequence voltage variation value; in the case where the fault current of the single-phase grounding fault is difficult to capture, accurate starting conditions are obtained, and the accuracy of the subsequent fault identification and phase selection process is further improved.

Description

Technical Field

[0001] The present invention belongs to the field of single-phase grounding fault processing, and in particular relates to a single-phase grounding fault feature recognition and phase selection method. Background Art

[0002] A low-current grounding system in a distribution network refers to a three-phase system in which the neutral point is ungrounded, the neutral point is grounded via an arc suppression coil, or the neutral point is grounded via a high-resistance grounding. In the event of a single-phase grounding fault, the three-phase line voltage remains essentially unchanged, and the fault current is low, making it difficult to identify and address.

[0003] Currently, a wide range of methods have been proposed for detecting low-current ground faults, including the zero-sequence component method, the transient power direction method, the injection method, and the traveling wave method. However, these methods all have limitations: they are influenced by human experience and are subject to interference from changes in distribution network line topology and environmental factors, resulting in low accuracy in identifying single-phase ground faults. Emerging artificial intelligence technologies, such as those used in single-phase ground fault detection, also rely on extensive training with historical data, making them impractical given current state of the art.

[0004] In addition, the existing technology uses similarity calculations on the currents in the distribution lines to determine the operating status of the power grid. However, in a single-phase grounding fault, due to the short occurrence time and small variation of the fault current and the objective current collection limitations, it is impossible to accurately identify the characteristics of a single-phase grounding fault by judging the similarity of the current. Summary of the Invention

[0005] In order to solve the problems of low accuracy in line selection for single-phase grounding faults in current distribution networks and insufficient ability to identify characteristics of minor faults, the present invention provides a single-phase grounding fault feature identification and phase selection method that can accurately identify single-phase grounding faults, precisely locate the fault phase, and improve the accuracy of grounding line selection.

[0006] A single-phase ground fault feature recognition and phase selection method includes the following steps:

[0007] S1: Obtain a zero-sequence voltage mutation value; the zero-sequence voltage is a self-generated zero-sequence voltage;

[0008] S2: Obtaining the effective value of the zero-sequence component; the effective value of the zero-sequence component includes the effective value of the self-generated zero-sequence voltage and current or the effective value of the external zero-sequence voltage and current;

[0009] S3: Based on the absolute value of the self-generated zero-sequence voltage or the external zero-sequence voltage of each branch of the distribution network, a zero-sequence voltage positive sequence is constructed to obtain the first similarity of the zero-sequence voltage positive sequences between the two branches. In combination with the zero-sequence voltages of the corresponding branches, a pairwise comparison is performed to obtain the zero-sequence voltage shape difference value to construct the zero-sequence voltage variation value of the fault line. Based on the zero-sequence voltage variation value, it is determined whether a branch is grounded and line selection is initiated;

[0010] S4: After determining the start-up, when the line status is normal operation, determine whether the zero-sequence voltage mutation or the zero-sequence voltage RMS exceeds the limit for start-up based on the preset value; calculate the zero-sequence reactive power; determine whether the fault is within the zone by the polarity of the zero-sequence reactive power; confirm the start-up in the event of an internal fault and begin the alarm or trip delay; the zero-sequence voltage RMS is the self-generated zero-sequence voltage RMS or the external zero-sequence voltage RMS;

[0011] After starting, if the selected line status is not running normally, find the reference point from the starting point, calculate the mutation amount of the three-phase current sampling data for half the sampling period after the reference point, and select the phase with the largest cycle-to-cycle mutation amount as the grounded phase.

[0012] S5: When the action conditions are met, output alarm or trip signal and phase selection result.

[0013] Furthermore, in S2, the external zero-sequence voltage and current effective value includes an external zero-sequence current effective value and an external zero-sequence voltage effective value. The external zero-sequence current effective value is directly collected using a zero-sequence current transformer CT, and the external zero-sequence voltage effective value is directly collected using a zero-sequence voltage transformer PT.

[0014] Furthermore, the self-generated zero-sequence voltage and current effective value in S2 includes the self-generated zero-sequence voltage effective value and the self-generated zero-sequence current effective value;

[0015] The zero-sequence voltage sampling value is obtained by adding the sampling values ​​of the three-phase voltage, and then the zero-sequence voltage sampling value is processed by the full-cycle Fourier differential filtering algorithm to obtain the self-generated zero-sequence voltage effective value;

[0016] The zero-sequence current sampling value is obtained by adding the sampling values ​​of the three-phase current, and then the zero-sequence current sampling value is processed by the full-cycle Fourier differential filtering algorithm to obtain the self-generated zero-sequence current effective value.

[0017] Furthermore, S3 is specifically:

[0018] S31: Construct the zero-sequence voltage forward sequence:

[0019] Obtain the self-generated zero-sequence voltage or external zero-sequence voltage of each branch of the distribution line at the current sampling time, take the absolute value and store it in a two-dimensional array in a loop. Each row of the two-dimensional array represents the absolute value of the zero-sequence voltage of a branch at different sampling times, that is, the positive sequence of the zero-sequence voltage; each column of the two-dimensional array represents the absolute value of the zero-sequence voltage of each branch at the same time;

[0020] S32: Obtain the first similarity between any two branches in the zero-sequence voltage forward sequence:

[0021] The similarity between the positive sequences of zero-sequence voltages of any two branches is recorded as the first similarity;

[0022] S33: Calculate the difference in zero-sequence voltage shape between any two branches:

[0023] After cyclically storing the two-dimensional array, the difference in the absolute value of the zero-sequence voltage at the same sampling time is calculated based on the zero-sequence voltage of any two branches, and the number of occurrences of each difference value is counted. The difference values ​​that appear infrequently are discarded, and the sum of all remaining differences is marked as the zero-sequence voltage shape difference value of the two branches;

[0024] S34: Calculate the zero-sequence voltage variation of each branch: expressed as:

[0025] ;

[0026] Where: is the zero-sequence voltage variation value of the i-th branch; n is the total number of branches of the distribution line; is the first similarity between the zero-sequence voltages of the i-th branch and the k-th branch; is the difference in zero-sequence voltage shape between the i-th branch and the k-th branch; To prevent decimals with a denominator of 0;

[0027] S35: Confirm start:

[0028] Based on the zero-sequence voltage variation value of each branch, the average value is taken according to the data of the branch. When the zero-sequence voltage variation value of a branch exceeds the limit and the corresponding average value is the largest, it is determined that the line grounding selection is started.

[0029] Furthermore, in S4, the starting constant of the zero-sequence voltage effective value takes a typical value of 8V, and the constant of the zero-sequence voltage mutation amount takes a typical value of 5V.

[0030] Furthermore, in S4, whether it is an intra-zone fault is determined by the polarity of the zero-sequence reactive power, specifically:

[0031] The zero-sequence reactive power is calculated at the moment of grounding. When the zero-sequence reactive power is greater than 0, it is determined to be an internal fault. When the zero-sequence reactive power is less than 0, it is determined to be an external fault.

[0032] Furthermore, in step S4, the reference point is found from the starting point, the mutation amount is calculated point by point for the three-phase current sampling data of half a sampling period after the reference point, and the phase with the largest cycle-to-cycle mutation amount among the three phases is selected as the grounded phase. Specifically, the process is as follows:

[0033] S41: Data storage:

[0034] After the grounding line selection start confirmation is detected, n sampling points are pushed forward from the starting point, and the three-phase current sampling values ​​​​before 2T are stored in the array for standby use with this sampling point as the reference point, where T represents the sampling period;

[0035] S42: Calculation of three-phase current mutation:

[0036] Calculate the three-phase current sampling values ​​of the length T / 2 after the reference point point by point; first obtain the current point sampling value, then obtain the sampling values ​​1T ago and 2T ago from the array; calculate the difference between the current point mutation value and the point mutation value 1T ago, that is, the cycle mutation value;

[0037] S43: Get the maximum value of the three-phase current mutation:

[0038] Within the data T / 2 after the reference point, calculate the cycle-to-cycle mutation of the three-phase currents A, B, and C point by point, and obtain the maximum cycle-to-cycle mutation of each phase;

[0039] S44: Select the grounded phase:

[0040] Compare the maximum cycle-to-cycle mutation values ​​of each phase and select the phase corresponding to the maximum value as the grounded phase.

[0041] Furthermore, the calculation formula for the mutation amount of phase A in S43 is as follows:

[0042] ;

[0043] ;

[0044] ;

[0045] Where: is the current sampling value of phase A current; is the sampling value of phase A current 1T ago; is the sampling value of phase A current 2T ago; is the modulus of the current point mutation of phase A current; is the modulus of the point mutation quantity of phase A current 1T before; is the weekly mutation amount of the current point, when When it is less than 0, Assign a value of 0.

[0046] The beneficial effects of the present invention are:

[0047] The present invention constructs a zero-sequence voltage positive sequence in the process of starting judgment of a single-phase grounding fault, thereby obtaining the first similarity of the zero-sequence voltage positive sequences between two branches, and compares the zero-sequence voltages of the corresponding branches in pairs to obtain zero-sequence voltage shape difference values ​​to construct the zero-sequence voltage variation value of the fault line, and determines whether a branch is started according to the zero-sequence voltage variation value; in the case where the fault current of a single-phase grounding fault is difficult to capture, accurate starting conditions are obtained, and it is not affected by line topology changes and environmental factors, which greatly improves power supply reliability and further improves the accuracy of subsequent fault identification and phase selection processes.

[0048] In actual testing, the solution of the present invention passed grounding waveform inversion and dynamic model testing. In particular, in dynamic model testing, this method achieved an accuracy rate of over 98% for operation and phase selection in hundreds of test scenarios, including stability faults of overhead lines and cable lines in systems with ungrounded neutral points, arc suppression coil grounding systems, and low-resistance grounding systems, as well as intermittent faults, intermittent arc grounding faults, PT disconnection, inconsistent CT ratios, and reversed CT polarity. This method significantly outperforms currently used single-phase grounding line selection methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The present invention is a flow chart of a single-phase grounding fault feature identification and phase selection method. DETAILED DESCRIPTION

[0050] The following are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or replacements that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the scope of protection of the present invention. The embodiments described below are only used to explain the present invention and cannot be interpreted as limiting the present invention. The scope of protection of the present invention should be based on the scope of protection of the claims. The embodiments of the present invention are described in detail below. In order to facilitate the description of the present invention and simplify the description, the technical terms used in the description of the present invention should be interpreted broadly, including but not limited to conventional replacement schemes not mentioned in this application, and also including direct implementation and indirect implementation.

[0051] Combine Figure 1 This embodiment describes a method for identifying characteristics of a single-phase grounding fault and selecting a phase, including the following steps:

[0052] like Figure 1As shown, the first step is to determine whether the grounding line selection function is enabled. The program determines this based on the status of the soft pressure plate and the control word. When both are enabled, the grounding line selection function is enabled. The second step is to determine whether the line selection function is blocked. The device is blocked by PT resonance and PT disconnection. If the blocking condition is not met, the grounding line selection function can be started normally.

[0053] Determine whether the device is in the startup state. If any of the following conditions is detected: zero-sequence voltage mutation exceeding the limit, zero-sequence voltage exceeding the limit, or grounding selection has been started, the program will confirm that it is in the startup state and can perform fault feature identification, phase selection, action, return, etc. Otherwise, the program will confirm that it is in the normal operation state and return after initialization.

[0054] To determine fault occurrence and phase selection, the system calculates zero-sequence reactive power (using internally generated or externally connected zero-sequence power, which can be enabled or disabled via control word) and three-phase current surges. A positive zero-sequence reactive power indicates an internal fault; a negative zero-sequence reactive power indicates an external fault. The fault phase with the largest three-phase current surge is selected, and the phase selection result is output along with the trip / alarm signal.

[0055] A method for identifying single-phase grounding fault characteristics and selecting phases, in one example, includes:

[0056] S1: Obtain the zero-sequence voltage mutation amount; the zero-sequence voltage is the self-generated zero-sequence voltage; specifically:

[0057] Synthesize self-generated zero-sequence voltage through three-phase voltage sampling channels:

[0058]

[0059] Where: is the self-produced zero-sequence voltage, is the sampling value of phase A voltage, is the B phase voltage sampling value, is the C phase voltage sampling value.

[0060] Calculate the self-produced zero-sequence voltage mutation amount using the self-produced zero-sequence voltage:

[0061] ;

[0062] ;

[0063] ;

[0064] Where: is the current sampling value of the self-produced zero-sequence voltage, is the sampling value of the self-produced zero-sequence voltage 1T before, is the sampling value of the self-produced zero-sequence voltage 2T before, is the current point mutation value of the self-produced zero-sequence voltage, is the point mutation amount of the self-produced zero-sequence voltage 1T before, It is the weekly mutation amount of the self-generated zero-sequence voltage at the current point.

[0065] S2: Obtain the effective value of the zero-sequence component; the effective value of the zero-sequence component includes the effective value of the self-generated zero-sequence voltage and current or the effective value of the external zero-sequence voltage and current; specifically:

[0066] The zero-sequence voltage sampling value is obtained by adding the sampling values ​​of the three-phase voltage, and then the zero-sequence voltage sampling value is processed by the full-cycle Fourier differential filtering algorithm to obtain the self-generated zero-sequence voltage effective value;

[0067] The zero-sequence current sampling value is obtained by adding the sampling values ​​of the three-phase current, and then the zero-sequence current sampling value is processed by the full-cycle Fourier differential filtering algorithm to obtain the self-generated zero-sequence current effective value.

[0068] The effective value of the self-generated zero-sequence voltage is calculated and expressed as:

[0069] ;

[0070] ;

[0071] ;

[0072] Where: is the effective value of the self-produced zero-sequence voltage, is the A-phase component of the self-generated zero-sequence voltage, is the B-phase component of the self-generated zero-sequence voltage, is the C-phase component of the self-generated zero-sequence voltage, is the real part of the self-generated zero-sequence voltage, is the real part of the A-phase component, is the real part of the B-phase component, is the real part of the C-phase component, is the imaginary part of the self-generated zero-sequence voltage, is the imaginary part of the A-phase component, is the imaginary part of the B-phase component, is the imaginary part of the C-phase component;

[0073] The calculation method for the effective value of self-produced zero-sequence current is the same as that for self-produced zero-sequence voltage. Please refer to the above formula.

[0074] The external zero-sequence voltage and current effective value includes an external zero-sequence current effective value and an external zero-sequence voltage effective value. The external zero-sequence current effective value is directly collected by a zero-sequence current transformer CT, and the external zero-sequence voltage effective value is directly collected by a zero-sequence voltage transformer PT.

[0075] S3: Based on the absolute value of the self-generated zero-sequence voltage or the external zero-sequence voltage of each branch of the distribution network, a zero-sequence voltage positive sequence is constructed to obtain the first similarity of the zero-sequence voltage positive sequence between the two branches. In combination with the zero-sequence voltages of the corresponding branches, the zero-sequence voltage shape difference value is obtained to construct the zero-sequence voltage variation value of the fault line. According to the zero-sequence voltage variation value, it is determined whether a branch is grounded and the line selection is started.

[0076] Furthermore, S3 is specifically:

[0077] S31: Construct the zero-sequence voltage forward sequence:

[0078] Obtain the self-generated zero-sequence voltage or external zero-sequence voltage of each branch of the distribution line at the current sampling time, take the absolute value and store it in a two-dimensional array in a loop. Each row of the two-dimensional array represents the absolute value of the zero-sequence voltage of a branch at different sampling times, that is, the positive sequence of the zero-sequence voltage; each column of the two-dimensional array represents the absolute value of the zero-sequence voltage of each branch at the same time;

[0079] S32: Obtain the first similarity between any two branches in the zero-sequence voltage forward sequence:

[0080] The similarity between the positive sequences of zero-sequence voltages of any two branches is recorded as the first similarity;

[0081] S33: Calculate the difference in zero-sequence voltage shape between any two branches:

[0082] After cyclically storing the two-dimensional array, the difference in the absolute value of the zero-sequence voltage at the same sampling time is calculated based on the zero-sequence voltage of any two branches, and the number of occurrences of each difference value is counted. The difference values ​​that appear infrequently are discarded, and the sum of all remaining differences is marked as the zero-sequence voltage shape difference value of the two branches;

[0083] S34: Calculate the zero-sequence voltage variation of each branch: expressed as:

[0084] ;

[0085] Where: is the zero-sequence voltage variation value of the i-th branch; n is the total number of branches of the distribution line; is the first similarity between the zero-sequence voltages of the i-th branch and the k-th branch; is the difference in zero-sequence voltage shape between the i-th branch and the k-th branch; To prevent decimals with a denominator of 0;

[0086] S35: Confirm start:

[0087] Based on the zero-sequence voltage variation value of each branch, the average value is taken according to the data of the branch. When the zero-sequence voltage variation value of a branch exceeds the limit and the corresponding average value is the largest, it is determined that the line grounding selection is started.

[0088] S4: After the start is determined, when the line selection state is normal operation; according to the preset set value, determine whether the zero-sequence voltage mutation amount or the zero-sequence voltage effective value exceeds the limit to start. When either of the two exceeds the limit or the grounding line selection function has been started, the grounding determination begins, otherwise exit; the zero-sequence voltage start set value and the zero-sequence voltage mutation amount set value are both empirical values, respectively, the start set value of the zero-sequence voltage effective value takes a typical value of 8V, and the set value of the zero-sequence voltage mutation amount takes a typical value of 5V. When any of the three conditions, namely, the zero-sequence voltage mutation amount exceeds the limit, the zero-sequence voltage exceeds the limit, and the device has been started, it means that the prerequisites for grounding start have been met, and the start, action, etc. can be determined. When none of the three conditions are met, it means that the current normal operation can be directly exited. When the zero-sequence voltage effective value exceeds the limit, the action determination begins; the zero-sequence reactive power is calculated; the zero-sequence reactive power polarity is used to determine whether it is an internal fault; when there is an internal fault, the start is confirmed, and the alarm or trip delay begins; among them, the zero-sequence voltage effective value is the self-generated zero-sequence voltage effective value or the external zero-sequence voltage effective value;

[0089] Based on the characteristics of single-phase grounding fault, the zero-sequence voltage and zero-sequence current effective values ​​are selected as the basic condition to exceed the limit. The zero-sequence reactive power at the current point is calculated through the sampling channels of zero-sequence voltage and zero-sequence current.

[0090] The limit violation is expressed as:

[0091] ;

[0092] ;

[0093] Where: is the effective value of self-produced / external zero-sequence voltage, is the effective value of self-generated / zero-sequence current, is the starting value of zero sequence voltage, It is the starting value of zero-sequence current, and the zero-sequence current value is 0.03A.

[0094] Internal and external faults are determined based on the polarity of zero-sequence reactive power: When zero-sequence reactive power is greater than 0, it is considered an internal fault; when zero-sequence reactive power is greater than 0, it is considered an internal fault. To prevent false tripping, a threshold of 0.4Var is set for zero-sequence reactive power (Var represents the unit of reactive power, also known as Var). Only when the zero-sequence reactive power calculated at four consecutive sampling points exceeds this threshold is it considered an internal fault, the system confirms the start, sets the start flag, and initiates a delayed alarm or trip.

[0095] If the zero-sequence voltage does not exceed the limit, the judgment is returned.

[0096] Furthermore, after S4 determines that the line status is not operating normally after starting, it finds the reference point forward from the starting point, calculates the sudden change amount of the three-phase current sampling data of T / 2 length after the reference point, and selects the phase with the largest sudden change amount as the grounded phase; specifically:

[0097] S41: Data storage:

[0098] After the grounding line selection start is confirmed, since the condition of the zero-sequence reactive power of 4 consecutive sampling points being greater than 0.4Var is used to determine the grounding line selection start confirmation, 4 points are pushed forward from the starting point, and the starting point is confirmed as the reference point, and the three-phase current sampling values ​​2T in front of it are stored in the array for standby use.

[0099] S42: Calculation of three-phase current mutation:

[0100] In a single-phase ground fault, we assume a stable operating state within 2T of the selected reference point. For the sampling values ​​within T / 2 after the reference point, the calculated mutation value of the previous cycle is bound to approach 0. However, the mutation value of the current point should be much larger than the mutation value of the previous cycle, and the body should show obvious single-phase ground fault transient characteristics. Therefore, the calculation of the cycle-to-cycle mutation value adopts the direct subtraction method without taking the modulus value, and the points less than 0 are directly assigned 0 and discarded.

[0101] S43: Get the maximum value of the three-phase current mutation:

[0102] Within the data T / 2 after the reference point, the cycle-to-cycle mutations of the three-phase currents A, B, and C are calculated point by point. For an intra-zone fault, the mutation of the faulted phase is much larger than that of the unfaulted phase. However, for an extra-zone fault, the magnitude and direction of the three-phase current mutations are essentially the same. Therefore, after calculating the cycle-to-cycle mutations of the three phases at the current point, we can first compare them. If the mutation of the largest phase is at least m times that of the other two phases (a typical value of m is 2), this is considered typical intra-zone fault data. The cycle-to-cycle mutation of the three-phase current at the current point is compared with the maximum cycle-to-cycle mutation of each channel, and the larger value is retained as the new maximum mutation value for that channel. If the set conditions are not met, the cycle-to-cycle mutation of the three phases at the current point is considered subtypical and discarded. The calculation then exits and waits for the next point, ultimately obtaining the maximum cycle-to-cycle mutation of each phase.

[0103] For example, the calculation formula for the mutation amount of phase A in S43 is as follows:

[0104] ;

[0105] ;

[0106] ;

[0107] Where: is the current sampling value of phase A current; is the sampling value of phase A current 1T ago; is the sampling value of phase A current 2T ago; is the modulus of the current point mutation of phase A current; is the modulus of the point mutation quantity of phase A current 1T before; is the weekly mutation amount of the current point, when When it is less than 0, Assign a value of 0.

[0108] S44: Select the grounded phase:

[0109] Compare the maximum cycle-to-cycle mutation values ​​of each phase and select the phase corresponding to the maximum value as the grounded phase.

[0110] S5: When the action conditions are met, output alarm or trip signal and phase selection result.

Claims

1. A method for identifying and selecting single-phase grounding fault characteristics, characterized in that: The steps include: S1: Obtain a zero-sequence voltage mutation value; the zero-sequence voltage is a self-generated zero-sequence voltage; S2: Obtaining the effective value of the zero-sequence component; the effective value of the zero-sequence component includes the effective value of the self-generated zero-sequence voltage and current or the effective value of the external zero-sequence voltage and current; S3: Based on the absolute value of the self-generated zero-sequence voltage or the external zero-sequence voltage of each branch of the distribution network, a zero-sequence voltage positive sequence is constructed to obtain the first similarity of the zero-sequence voltage positive sequences between the two branches. In combination with the zero-sequence voltages of the corresponding branches, a pairwise comparison is performed to obtain the zero-sequence voltage shape difference value to construct the zero-sequence voltage variation value of the fault line. Based on the zero-sequence voltage variation value, it is determined whether a branch is grounded and line selection is initiated; S4: After determining the start-up, when the line status is normal operation, determine whether the zero-sequence voltage mutation or the zero-sequence voltage RMS exceeds the limit for start-up based on the preset value; calculate the zero-sequence reactive power; determine whether the fault is within the zone by the polarity of the zero-sequence reactive power; confirm the start-up in the event of an internal fault and begin the alarm or trip delay; the zero-sequence voltage RMS is the self-generated zero-sequence voltage RMS or the external zero-sequence voltage RMS; After starting, if the selected line status is not running normally, find the reference point from the starting point, calculate the mutation amount of the three-phase current sampling data for half the sampling period after the reference point, and select the phase with the largest cycle-to-cycle mutation amount as the grounded phase. S5: When the action conditions are met, output alarm or trip signal and phase selection result; S3 specifically: S31: Construct the zero-sequence voltage forward sequence: Obtain the self-generated zero-sequence voltage or external zero-sequence voltage of each branch of the distribution line at the current sampling time, take the absolute value and store it in a two-dimensional array in a loop. Each row of the two-dimensional array represents the absolute value of the zero-sequence voltage of a branch at different sampling times, that is, the positive sequence of the zero-sequence voltage; each column of the two-dimensional array represents the absolute value of the zero-sequence voltage of each branch at the same time; S32: Obtain the first similarity between any two branches in the zero-sequence voltage forward sequence: The similarity between the positive sequences of zero-sequence voltages of any two branches is recorded as the first similarity; S33: Calculate the difference in zero-sequence voltage shape between any two branches: After cyclically storing the two-dimensional array, the difference in the absolute value of the zero-sequence voltage at the same sampling time is calculated based on the zero-sequence voltage of any two branches, and the number of occurrences of each difference value is counted. The difference values ​​that appear infrequently are discarded, and the sum of all remaining differences is marked as the zero-sequence voltage shape difference value of the two branches; S34: Calculate the zero-sequence voltage variation of each branch: expressed as: ； Where: is the zero-sequence voltage variation value of the i-th branch; n is the total number of branches of the distribution line; is the first similarity between the zero-sequence voltages of the i-th branch and the k-th branch; is the difference in zero-sequence voltage shape between the i-th branch and the k-th branch; To prevent decimals with a denominator of 0; S35: Confirm start: Based on the zero-sequence voltage variation value of each branch, the average value is taken according to the data of the branch. When the zero-sequence voltage variation value of a branch exceeds the limit and the corresponding average value is the largest, it is determined that the line grounding selection is started; In step S4, the reference point is found from the starting point, the mutation amount of the three-phase current sampling data of half a sampling period after the reference point is calculated point by point, and the phase with the largest cycle-by-cycle mutation amount among the three phases is selected as the grounded phase. The specific process is as follows: S41: Data storage: After the grounding line selection start confirmation is detected, n sampling points are pushed forward from the starting point, and the three-phase current sampling values ​​​​before 2T are stored in the array for standby use with this sampling point as the reference point, where T represents the sampling period; S42: Calculation of three-phase current mutation: Calculate the three-phase current sampling values ​​of the length T / 2 after the reference point point by point; first obtain the current point sampling value, then obtain the sampling values ​​1T ago and 2T ago from the array; calculate the difference between the current point mutation value and the point mutation value 1T ago, that is, the cycle mutation value; S43: Get the maximum value of the three-phase current mutation: Within the data T / 2 after the reference point, calculate the cycle-to-cycle mutation of the three-phase currents A, B, and C point by point, and obtain the maximum cycle-to-cycle mutation of each phase; S44: Select the grounded phase: Compare the maximum cycle-to-cycle mutation values ​​of each phase and select the phase corresponding to the maximum value as the grounded phase; The calculation formula for the mutation amount of phase A in S43 is as follows: ； ； ； Where: is the current sampling value of phase A current; is the sampling value of phase A current 1T ago; is the sampling value of phase A current 2T ago; is the modulus of the current point mutation of phase A current; is the modulus of the point mutation quantity of phase A current 1T ago; is the weekly mutation amount of the current point, when When it is less than 0, Assign a value of 0.

2. A single-phase grounding fault feature identification and phase selection method according to claim 1, characterized in that: In S2, the external zero-sequence voltage and current effective value includes an external zero-sequence current effective value and an external zero-sequence voltage effective value. The external zero-sequence current effective value is directly collected using a zero-sequence current transformer CT, and the external zero-sequence voltage effective value is directly collected using a zero-sequence voltage transformer PT.

3. A single-phase grounding fault feature identification and phase selection method according to claim 1, characterized in that: The self-generated zero-sequence voltage and current effective value in S2 includes the self-generated zero-sequence voltage effective value and the self-generated zero-sequence current effective value; The zero-sequence voltage sampling value is obtained by adding the sampling values ​​of the three-phase voltage, and then the zero-sequence voltage sampling value is processed by the full-cycle Fourier differential filtering algorithm to obtain the self-generated zero-sequence voltage effective value; The zero-sequence current sampling value is obtained by adding the sampling values ​​of the three-phase current, and then the zero-sequence current sampling value is processed by the full-cycle Fourier differential filtering algorithm to obtain the self-generated zero-sequence current effective value.

4. A single-phase grounding fault feature identification and phase selection method according to claim 1, characterized in that: In S4, the starting constant of the zero-sequence voltage effective value takes a typical value of 8V, and the constant of the zero-sequence voltage mutation amount takes a typical value of 5V.

5. A single-phase grounding fault feature identification and phase selection method according to claim 1, characterized in that: In S4, the zero-sequence reactive power polarity is used to determine whether the fault is an internal fault. Specifically: The zero-sequence reactive power is calculated at the moment of grounding. When the zero-sequence reactive power is greater than 0, it is determined to be an internal fault. When the zero-sequence reactive power is less than 0, it is determined to be an external fault.

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